We are developing a method for the determination of phosphorus in Haemophilus b polysaccharide conjugate vaccines by inductively coupled plasma-atomic emission spectroscopy (ICP- AES). The concentration of polysaccharide is directly related to the concentration of phosphorus measured by ICP-AES. Phosphorus is present in the polyribosylribitol phosphate (PRP) group of the Haemophilus b polysaccharide conjugate vaccine. The Haemophilus b polysaccharide conjugate vaccine is analyzed for phosphorus content within the range of 0.67 to 1.01 ?g phosphorus per 0.5 ml dose.